The present invention relates to a control rod for controlling the power of a boiling water reactor and a method for manufacturing the same.
The control rod typically has a structure wherein a handle is attached to an axially upper part of a tie rod having a substantially cruciform cross section; a lower part support member (or velocity limiter) is attached to at an axially lower part of the tie rod; and four sheaths, each of which incorporates a reaction rate controlling material, are fixed at a lower end of the handle, an upper end of the lower part support member and ends of the substantially cruciform of the tie rod by welding. In this case, a perfect weld penetration by the TIG (tungsten inert gas) welding has been performed for welding the sheaths to the ends of the handle, the lower part support member and the tie rod.
The control rod moves upward and downward in the narrow gap secured among the fuel assemblies during operation of the reactor. Therefore, a high degree of machining precision is required in manufacturing the control rod.
However, the conventional TIG welding has such drawbacks that it requires a large amount of heat input and tends to increase deformation due to welding. Thus, in order to suppress the deformation caused by welding, a method employing a laser welding, which requires a less amount of heat input, has been proposed in Japanese Patent Laid-open No. 2000-329885.
In the aforementioned prior art literature, the perfect weld penetration is carried out in the following manner. Steps are provided on each of tips of arms of the tie rod to fit a U-shaped tip of each of the sheaths thereonto, and each of tips of the sheath are directly irradiated with a laser beam in such a manner that the axial center position of the beam is shifted from an end face of the step of the tie rod to a side opposite to the axis center of the tie rod by 0.1 to 2.0 mm.
The above-described prior art has the following problems. Specifically, since a width of the step at the tip of each of the arms of the tie rod is typically about 0.5 mm, an overlap of the step of the tie rod with the tip of the sheath is about 0.5 mm. Therefore, if an error occurs in the axial center position of the laser beam and the laser beam is deviated from the very narrow overlap portion, the sheath is heated to melt down due to a slow heat transfer of the laser beam to the tie rod, to thereby cause a welding failure.
Further, even if the irradiation position of the laser beam is located within the overlap, a contact area of the step of the tie rod with the tip of the sheath must be sufficiently large to satisfactorily transfer the heat of the laser beam to the tie rod. Therefore, the step of the tie rod must be machined to achieve a precise rectangular shape. More specifically, if an R shape (round portion) is formed at a corner of the step of the tie rod, the contact area of the tie rod with the sheath becomes small to fail to provide the satisfactory heat transfer, and such imprecision may cause the melt-down of the sheath, resulting in the welding failure.
In order to prevent the above problems, the prior art requires a high precision control of the laser beam for the prevention of the deviation of the laser beam irradiating position from the overlap and a high precision machining of the step of the tie rod. Thus, it has been difficult to simplify the manufacturing process of the control rod, and, also, the production cost has been undesirably increased in the prior art.